3d piezoelectric polymer materials and devices

ABSTRACT

Methods, systems, and devices are disclosed for fabricating 3D piezoelectric materials. In one aspect, a method includes photopolymerizing a selected portion of a two dimensional plane in a sample of a photoliable polymer solution containing piezoelectric nanoparticles to form a layer of a piezoelectric material, the photopolymerizing including directing light from a light source based on a pattern design in the selected portion of the photoliable polymer solution; and moving one or both of the sample and the directed light to photopolymerize another selected portion of another two dimensional plane in the sample to form another layer of the piezoelectric material.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is timely filed to claim the priority andbenefits of U.S. Provisional Patent Application No. 62/155,388, filedApr. 30, 2015, entitled “3D FABRICATION METHODS OF PIEZOELECTRIC POLYMERCOMPOSITE MATERIALS.” The entire content of the before-mentioned patentapplication is incorporated by reference as part of the disclosure ofthis application.

TECHNICAL FIELD

This patent document relates to systems, devices, and processes havingspecialized piezoelectric materials.

BACKGROUND

Piezoelectricity is a material property where electric chargeaccumulates in response to applied mechanical stress on the material.Example of materials that exhibit piezoelectricity can include certaincrystals, ceramics, and biological matter, e.g., such as DNA, variousproteins, and bone. The piezoelectric effect is a linearelectromechanical interaction between the mechanical and the electricalstate in crystalline materials with no inversion symmetry. Thepiezoelectric effect is a reversible process. The direct piezoelectriceffect is the internal generation of electrical charge resulting from anapplied mechanical force, and the converse piezoelectric effect is theinternal generation of a mechanical strain resulting from an appliedelectrical field.

SUMMARY

A valuable property of piezoelectric materials is an ability to convertcompressive/tensile stresses to an electric charge, or vice versa. Anexample of a piezoelectric material is a brittle ceramic known as leadzirconate titanate. Most piezoelectric materials in systems are based onbrittle ceramics such as lead zirconate titanate. Another example of apiezoelectric material is a piezoelectric polymer such as materials inthe polyvinylidene fluoride group that have good piezoelectric polymerperformance. However, conventional techniques do not offer simpleapproaches to fabricating 3D structures in piezoelectric polymers ormultilayered architectures which would open up infinite possibilities inthe design of more complicated device geometries. The disclosedtechnology facilitates a cheaper and easier fabrication of 3D structuresin piezoelectric polymers.

Disclosed are 3D manufacturing processes to manipulate piezoelectricpolymers into virtually any shape using light-assisted polymerization.Any light projection techniques including stereolithography or directphoto-printing techniques including laser-assisted two-photon 3Dprinting can be used to polymerize the piezoelectric materials. Anotherlight projection technique can include digital projection printing thatcan also be used to polymerize the piezoelectric materials. Thedisclosed methods can include using piezoelectric nanoparticles graftedto a photoliable polymer solution to manufacture biocompatible,mechanically flexible, size scalable, shape tunable, and highlyresponsive piezoelectric materials.

The subject matter described in this patent document can be implementedto provide one or more of the following features. For example, thepolymerization process links up the polymer matrix to piezoelectricnanoparticles, which enables 3D piezoelectric materials to be writtenwith control over the size and shape that is not afforded by any othertechnique.

Exemplary applications of the disclosed technology can include a widerange of devices, systems, and processes utilizing direct and/orconverse piezoelectric effects, e.g., non-volatile low voltage memory,loud speakers, acoustic imaging, energy harvesting, or electricalactuators, just to name a few. The disclosed technology can also beimplemented to print virtually any 3D piezoelectric shape, whilemaintaining a strong piezoelectric coefficient and biocompatibleproperties, and thus is applicable to biomimetic materials (e.g.,artificial skin, tympanic membrane), integratedmicro/nanoelectromechanical systems (MEMS/NEMS, e.g., such as mechanicalactuators), sensors (e.g., acoustic detection), bio-imaging (e.g., highresolution, compact ultrasonic imaging instruments), in vitro energyscavenging, and biometrics and security, among others.

A 3D piezoelectric material can have several useful applications. Forexample, a 3D piezoelectric polymer can be used in pressure sensingmouthguards for sports applications. For example, the disclosedtechnology can be used to provide embeddable, flexible pressure sensorswith real time data read out capabilities. The real time data from theembeddable and flexible pressure sensors can be read by a coach using anportable device, such as an iPad. When the real time reading detectsimpacts large enough to cause a concussion, the coach or other personnelcan be notified. Since the second hit after an initial injury to thehead can be extremely serious and even deadly, cheap but effectivepolymer-based piezoelectric material could be a viable way to monitorimpacts in sports, military, or other situations to reduce the chancesof serious injury. The polymer-based piezoelectric material could bestrategically placed for example in a helmet and when the materialgenerates an output due to an impact, which is a function of pressure,the information related to the output could be relayed to an observerwho would instantly know who received the hit, the location of the hit,and if the hit was at levels (assuming the transducers were calibrated)that could cause a traumatic brain injury.

In one implementation, a method is provided to fabricate a piezoelectricmaterial and includes photopolymerizing a selected portion of a twodimensional plane in a sample of a photoliable polymer solutioncontaining piezoelectric nanoparticles to form a layer of apiezoelectric material, the photopolymerizing including directing lightfrom a light source based on a pattern design in the selected portion ofthe photoliable polymer solution; and moving one or both of the sampleand the directed light to photopolymerize another selected portion ofanother two dimensional plane in the sample to form another layer of thepiezoelectric material.

In another implementation, a device is provided to include apiezoelectric material structure which includes a three-dimensionalstructure; and a plurality of piezoelectric nanoparticles grafted to thethree-dimensional structure. The piezoelectric material structure isoperable to generate an electric charge in response to an appliedmechanical force and to generate a mechanical force in response to anapplied electrical signal; and the piezoelectric material structure isfabricated using a light-assisted polymerization technique by causingcrosslinking in a photoliable polymer solution.

In yet another implementation, a method to fabricate a device based on apiezoelectric material is provided to include directing an optical beamto a photoliable polymer solution containing piezoelectricnanoparticles; modulating and controlling the optical beam in a way thatcauses photopolymerizing of selected portions of the photoliable polymersolution to form a three dimensional piezoelectric material structure;and forming one or more electrical contacts to the three dimensionalpiezoelectric material structure to construct the device to apply anelectrical control to the three dimensional piezoelectric materialstructure, or to receive an electrical response from the threedimensional piezoelectric material structure under a stress.

The above features and their implementations and other aspects of thedisclosed technology are described in greater detail in the drawings,the description and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a block diagram of an exemplary digital projectionprinting setup that projects dynamic digital masks or pattern design onthe photoliable polymer solution.

FIG. 1B shows an exemplary scanning electron micrograph of exemplarybarium titanate nanoparticles grown via a hydrothermal process.

FIG. 1C shows an illustrative diagram of exemplary piezoelectricmaterials with exemplary barium titanate nanoparticles grafted to apolyethylene glycol diacrylate matrix.

FIG. 1D shows an example of a fabrication system for 3D piezoelectricmaterial.

FIGS. 2A-2D show a collage of different microstructures including dotarrays, square arrays, and honeycomb arrays.

FIG. 3A shows an image of an exemplary 3D mushroom-like array grownusing dynamical optical projection stereolithography.

FIG. 3B shows an image of an exemplary microtubule shape.

FIG. 4A shows a data plot of the voltage response of photopolymerizedpiezoelectric material cycled under a load applied perpendicular to thesurface of the film.

FIG. 4B shows a data plot of the effective piezoelectric modulus as afunction of barium titanate mass loading.

FIG. 5 show schematic diagrams of an exemplary photofabrication methodof the disclosed technology to produce piezoelectric materials

FIGS. 6A-6D show exemplary electron micrograph images and illustrativediagrams showing exemplary piezoelectric materials produced by thedisclosed methods.

FIG. 7 shows an electron micrograph of an exemplary piezoelectricmaterial fabricated into a 3D fingerprint structure.

FIG. 8 shows a schematic of the charge amplifier used in thepiezoelectric experiments

FIG. 9 shows pattern of the as-made BTO nanoparticles and a referenceBaTiO₃ pattern.

FIG. 10 shows chemical structure of TMSPM, and the FTIR spectra of pureTMSPM, as-made BTO nanoparticles, and TMSPM-grafted BTO nanoparticles.

FIG. 11 shows the UV-Visible spectra of pure PEGDA, and PEDGA/PTOsolutions of 1%, 5%, and 10%.

FIGS. 12A-12B show a schematic of the FlexiForce sensor (Tekscan)circuit and the calibration curve.

FIGS. 13A-13F show several exemplary applications for 3D piezoelectricmaterial.

DETAILED DESCRIPTION

The ability to convert compressive/tensile stresses to an electriccharge, or vice versa provides valuable property of piezoelectricmaterials in a wide range of applications that utilize the directpiezoelectric effect (e.g., mechanical stress forming an electric field)or converse piezoelectric effect (e.g., electric voltage forming amechanical deformation). Some examples of applications based onpiezoelectric materials include non-volatile low voltage memory, audiospeakers, acoustic imaging, in systems are based on brittle ceramicssuch as lead zirconate titanate (PZT) which has one of the highest knownpiezoelectric coefficients (e.g., d₃₃ greater than 300 pC/N; dependingon composite and processing conditions). Compared to PZT, piezoelectricpolymers offer smaller piezoelectric responses while also offeringseveral unique capabilities that make them ideal candidates for systemsthat require, for example, mechanical flexibility, smaller activeelements, biocompatibility, and processability. Such piezoelectricpolymers can include, for example, lead magnesium niobate-lead titanatePb(Mg_(1/3)Nb_(2/3))O₃—PbTiO₃ (PMN-PT; d₃₃ up to approximately 2500pC/N), or other perovskite-based oxides such as barium titanate (BTO;d₃₃ greater than 200 pC/N; depending on ceramic type and processingconditions).

Polyvinylidene fluoride (PVDF) is a pure polymer discovered in 1969 byKawai. PVDF has a piezoelectric coefficient (d₃₃ approximately −20 to−34 pC/N) that is over an order of magnitude smaller than PZT. Due toits excellent mechanical flexibility, biocompatibility, andsolution-based processability, it is actively being investigated forapplications including non-volatile low voltage memory, acoustictransducers, and implantable medical devices. PVDF materials have a goodpiezoelectric polymer performance, yet it is difficult to fabricatethese structures into individual active elements, complex architectures,or 3D patterns. Technologies in the area of micro- and nanofabricationof piezoelectric polymers can have an enormous impact on the developmentof biodiagnostics, nano- and microelectromechanical systems (NEMS/MEMS),imaging, sensors, and electronics.

Certain nano- and micro-fabrication techniques are currently availablefor producing ferroelectric and piezoelectric materials, e.g., includingelectron beam lithography, ion milling, soft lithography, self-assembly,electrospinning, and contact printing. However, existing implementationsof these techniques tend to be complex and many of existingimplementations do not offer simple approaches to fabricating 3Dstructures in piezoelectric polymers or multilayered architectures whichwould open up possibilities in the design of more complicated devicegeometries.

The technology disclosed in this patent document is based on the use ofphotoliable polymer solution and light projection techniques includingprojection stereolithography or direct photo-printing techniquesincluding laser-assisted two-photon 3D printing. 3D manufacturingprocesses are disclosed to manipulate piezoelectric polymers intovarious desired shapes, configurations or geometries usinglight-assisted polymerization. In some implementations,stereolithography or direct photo-printing techniques includinglaser-assisted two-photon 3D printing are used to polymerize thepiezoelectric materials.

Using light to polymerize the piezoelectric materials in photoliablesolutions can be achieved in different ways. For example, light at asufficient short wavelength such as UV wavelengths can be used toactivate and cause crosslinking in a photoliable solution by a singlephoton absorption process.

Alternatively, laser-assisted two-photon polymerization is generallybased on introducing laser energy to a photoliable solution at a precisespace for a precise period of time to meet the nonlinear opticalthreshold for the two-photon polymerization. By focusing laser pulses onselected locations of the photoliable solution, the pulses initiatetwo-photon polymerization via two-photon absorption so that thephotoliable solution is selectively crosslinked at certain locations toform a desired polymer structure while other parts of the photoliablesolution remains in their salutation state. In implementation of thistwo-photon polymerization process, a laser selected can be at a laserwavelength that is longer than the one or more UV laser wavelengthssuitable for causing the photoliable solution to crosslink via a singlephoton absorption so that such a longer laser wavelength would not beable to cause crosslink via a single photon absorption process withoutthe nonlinear two-photon absorption process. Two-photon absorption isthe simultaneous absorption of two photons to excite a molecule from oneenergy state to a higher energy state. Thus, laser-assisted two-photon3D printing can allow a user to determine and precisely control the sizeand shape of the polymerized material.

In the above and other optically caused polymerization processes, thelight beam is controlled spatially when illuminating the photoliablesolution to cause crosslinking and create a desired spatial pattern ofthe polymerization in the photoliable solution along the X, Y and Zdirections without causing crosslinking in other parts of thephotoliable solution. The light beam can be controlled in various waysto achieve this. For example, the light beam of a suitable UV wavelengthin a single photon absorption process or of a wavelength longer than theUV spectral range in a two-photon process can be spatially scanned whilethe optical power or intensity of the light beam is modulated based onthe scanned positions to crate the desired spatial pattern along the X,Y and Z directions.

Notably, a spatial light modulation device may be used to imprint atwo-dimensional spatial pattern across the light beam and this spatiallymodulated light beam can be projected into the photoliable solution tocause crosslinking and create the desired spatial pattern of thepolymerization in the photoliable solution. Depending on the size of thecross section of the light beam relative to the cross-sectional size ofthe desired spatial pattern to be formed in the photoliable solution,the spatially modulated light beam may be projected once to complete thepolymerization in the photoliable solution for the desired spatialpattern in a particular depth Z in the solution, or may be spatiallyscanned along X and Y directions to be projected into the photoliablesolution one or more times with different spatial modulation patterns tocomplete polymerization in the photoliable solution for the desiredspatial pattern in a particular depth Z in the solution. As a specificexample for this implementation, a digital projection printing (DPP) isused to polymerize the piezoelectric materials. The concept of digitalprojection printing (DPP) is generally based on introducing images of UVlight to photoliable solution. The images of UV light can be created bya digital micromirror-array device (DMD). The images of the UV lightpolymerize the photoliable solution at precise locations. This processis repeated to create additional layers of polymerized solution. Thedisclosed methods can include using piezoelectric nanoparticles graftedto a photoliable polymer solution to manufacture biocompatible,mechanically flexible, size scalable, shape tunable, and highlyresponsive piezoelectric materials.

An exemplary process for building 3D structures with stereolithographycan include the exposure of light (typically from a laser or lightemitting diode) to a photoliable liquid (e.g., polymer solution withacrylated monomer units) which creates cross-linked regions where thelight irradiates the matrix. Once a single layer is carved out with thelight, the sample is translated to allow the next layer to be written.The actual patterning can be programmed with Computer-Aided Design(CAD). In certain implementations, the spatial resolution can be limitedby certain factors, e.g., actinic radiation, free radical diffusion, andthe optical system designs and may be, in some systems, approximately 75to 250 μm in the x-y direction and about 100 μm in the z-direction. Thethroughput of the stereolithography process in certain implementationsmay be slow when such implementations use the point-by-point scanning inthe direct-write operation.

FIG. 1A illustrates an example a fabrication system for making 3Dpiezoelectric polymer structures or devices based on light-assistedpolymerization using UV light. To achieve higher throughput andresolutions using the basic concepts of stereolithography, microscaledigital projection printing (DPP) can be used to leverage a digitalmicromirror-array device (DMD) to produce a dynamic digital mask. Theprojected images from the DMD can be focused on the polymer solution andfeature sizes as small as 1 μm can be generated by sequentialpolymerization steps.

FIG. 1A shows a block diagram of the exemplary digital projectionprinting (DPP) setup (100) that uses, for example, UV light from a lightsource (101) to project dynamic digital masks or pattern design producedby digital micromirror array device (DMD) (110) on a selected portion ofthe photoliable polymer solution supplied to a polymer holding platform140 on which a desired 3D piezoelectric polymer structure is to beformed. For example, using the exemplary setup, any pattern can bedigitized and the digital micromirror device (DMD) (110) can project anydynamic digital masks or pattern design in form of a spatially modulatedUV light beam onto the polymer solution to polymerize a selected portionof the polymer solution at different locations based on the desiredpattern design carried in to the spatially modulated UV light beam 120.An optical imaging module 130 can be used to perform the optical beamforming and focusing to project the spatially modulated UV light beam120 into the polymer solution on the polymer holding platform 140. Thisprocess can be repeated in stages with same or different patterns topolymerize additional layers of the polymer solution to create, forexample, 3D structures.

In an exemplary implementation of the disclosed technology to produce 3Dphoto-writable piezoelectric materials, piezoelectric nanoparticles wereincorporated into a photoliable polymer solution, as shown in FIG. 1B.For example, after exposure to light, the liquid polymerizes andencapsulates the piezoelectric nanoparticles creating any 3D patterndefined by the user. FIG. 1B shows an exemplary scanning electronmicrograph of exemplary BTO nanoparticles grown via a hydrothermalprocess.

An exemplary system is described for the exemplary work that leveragesBTO nanoparticles embedded in a polyethylene glycol diacrylate (PEGDA)matrix, but the platform can be universal for other photoliable polymersolutions, e.g., including, but not limited to, [poly(methylmethacrylate), poly(acrylic acid), poly(lactic acid)] and piezoelectricnanoparticles (e.g., PZT, ZnO, PMN-PT, NaNbO₃). The BTO nanoparticleswere synthesized using, for example, hydrothermal processing thatcombines metal alkoxides such as Ti-butoxide (Ti[O(CH₂)₃CH₃]₄ with metalhydroxides such as Ba(OH)₂in an autoclave at 150-300° C. The meandiameter of the synthesized nanoparticles was 85±15 nm, as shown in FIG.1C.

In an exemplary system, the chemical interface between the piezoelectricnanoparticle and the polymer matrix can provide the piezoelectricmaterial with enhanced performance. For example, to enhance the stresstransfer efficiency from the matrix to the BTO nanoparticles and boostthe piezoelectric outputs of the fabricated materials, a linkermolecule, such as 3-trimethoxysilylpropyl methacrylate (TMSPM), can beused to covalently link the BTO surface to the PEGDA matrix.

FIG. 1C shows an illustrative diagram showing the exemplarypiezoelectric materials with the exemplary BTO nanoparticles (106)grafted to a PEGDA matrix (107). The exemplary zoomed section of thediagram shows the TMSPM linker (108) covalently linked to thenanoparticle surface and cross-linked with the PEGDA matrix (107).

Under light exposure the carbon-carbon double bonds of the TMSPMcross-link with the polymer matrix forming a strong bond between thepiezoelectric nanoparticles and polymer network. For example, comparedto other piezoelectric materials that utilize BTO nanoparticles embeddedin an elastomer (e.g., polydimethylsiloxane (PDMS)) with carbonnanotubes (CNTs) as a mechanical-to-electrical enhancer, the directgrafting of molecular linkers provides a simpler and more efficientroute to help funnel energy to the piezoelectric structures. Inaddition, for example, removing the CNTs significantly improves theoptical transparency of the material. Other nanocomposite polymers thathave also recently been demonstrated to produce strong piezoelectricoutputs without the need for additives include systems such asPDMS/PMN-PT nanowire composites. After mixing the surface-treated BTOnanoparticles with the PEGDA solution, a photoinitiator such as2,2-dimethoxy-2-phenylacetophenone (DMPA) or Irgacure 651 is added togenerate free radicals in regions where only light is exposed. Once freeradicals are formed they attack the carbon-carbon double bonds of themonomers in solution, producing acrylic monomers with free electronsthat attack other monomers forming oligomers and eventually a vastcross-linked network. The chain reaction propagates until two radicalsneutralize or the irradiation source is turned off. For the exemplaryDPP set-up described, the microstructure arrays were fabricated in veryshort times (e.g., times of less than 2 seconds), but this can be tunedby altering variables such as irradiation power, photoinitiatorconcentration, monomer concentration, nanoparticle loading, and/oradding a quencher.

FIG. 1D shows an example of a fabrication system from which FIG. 1A is aspecific implementation of this system. The spatial light modulationdevice 110 can be implemented in various forms for modulating the UVlight from the UV light source 101 to carry a desired light modulationcorresponding to a partial location (X, Y and Z) in the polymer solutioncarried by the platform 140. In the example in FIG. 1A, a DMD is used asthis spatial light modulator. In other implementations, other lightmodulation devices may be used. The motion control device 150 can becoupled to the spatial light modulator 110, the optical imaging module130, or the assembly formed by the spatial light modulator 110 and theoptical imaging module 130 that would control the relative positioningof the UV beam 120 with respect to the X, Y and Z positioning of thefocused UV light on the platform 140. The motion control device 150 canalso be coupled to the platform 140. In the position control along thevertical direction Z, the fabrication process may be performed at onelayer at a time in the polymer solution and subsequently polymerize thesubsequent layers of the polymer solution above a previously solidifiedlayer to form a final 3D structure.

By controlling the digital photomask, virtually any shape can beprojected onto the polymer solution and printed within seconds. FIGS.2A-2D show a collage of different microstructures, e.g., including dotarrays (FIG. 2A), square arrays with different sized void spaces (FIGS.2B and 2C), and honeycomb arrays (FIG. 2D) that were fabricated usingthe exemplary DPP apparatus coupled with a 365 nm light source. Althoughsimilar structures on pure polymers can be produced with otherfabrication methods such as contact printing, the photo-printing processcan be carried out over much larger areas with high reproducibility andfidelity. For 3D direct printing the stage can be translated in thez-direction (perpendicular to substrate surface) while the projectedimage is altered. The exemplary piezoelectric microstructures of FIGS.2A-2D were fabricated in less than 2 seconds using a PEGDA solutionloaded with 1% of the TMSPM-modified BTO nanoparticles.

FIG. 3A shows an image of an exemplary 3D mushroom-like array grownusing, for example, dynamic optical projection stereolithography. FIG.3B shows an image of an exemplary microtubule shape. For example, afterreleasing a film fabricated into a honeycomb array, the structure rollsup into a microtubule shape due to slight stress gradients in the films.

FIG. 3A shows an arbitrary mushroom-like array that has a smaller basediameter compared to the top. By focusing the projected light on a planein the liquid (or liquid surface), and synchronizing the stage movementwith the incremental change in the projected features, 3D structures canbe carved out with smooth side walls using, for example, a processcalled dynamic optical projection stereolithography.

For some exemplary structures, designs may require complex void regionslayered on top of each other or when structures need to be hollowed out.This may mainly be due to the polymer absorption process being activatedby single photons which limits how deep the light can be focused intothe material without initiating polymerization. Anywhere light hits thesample will create free radicals, so designs have to be optimized tolimit exposure times to regions of the liquid outside the focal area andoptical quenchers have to be used to lower the rate of free radicalformation in unwanted areas of the beam path. Nonlinear opticalprocesses such as two-photon absorption (TPA) can be employed tocircumvent the 3D printing issues of single photon absorption, but theremay be a trade off in fabrication time since TPA may requirepoint-by-point scanning.

These exemplary patterns demonstrate the ability to reach an exemplaryresolution of DPP (e.g., approximately 1 μm) with curved, adjoining,straight, and void regions. For example, since a composite material wasused, the resolution will be strongly dependent on the light-matterinteraction of the BTO nanoparticles. For example, in the exemplaryimplementations, the BTO mass loadings of 1-10% and 1 photoinitiatorproduced excellent transfer efficiencies of the mask to the solidpolymer structures while still offering strong piezoelectric outputs andsimilar mechanical properties to the pure PEGDA materials. FIG. 11 showsthe UV-Visible spectra of pure PEGDA (1101), and PEDGA/PTO solutions of1% (1102), 5% (1103), and 10% (1104). The transmittance is directlyrelated to the BTO loading fraction and shows higher transmission atlonger wavelength. When the loading goes above 10% the transparency ofthe polymer goes below 5% at 365 nm which washes out the projected maskand causes shape distortions similar to what is observed in overexposedphotoresists. Extinction spectra of the BTO nanoparticles, shown insubsequent figures, clearly showed the direct relationship betweenlight-matter interactions and the BTO concentration. For example, higherloading fractions should be tolerable if tighter light sources are used,the photopolymerization wavelength is tuned so that it falls in thehigher transmission (longer wavelengths) region of the colloidal polymersolution, or if TPA techniques are utilized.

The exemplary composite materials have to be activated in order tobecome piezoelectric. This can require that the dipoles in theperovskite crystallites be aligned using a poling field that is largerthan the coercive field (approximately 10 V/μm) of the BTOnanoparticles. In the exemplary implementations, this was achieved usingindium tin oxide (ITO) coated glass slides as the top and bottomelectrodes, which also served as the top and bottom surface of thephotofabrication cell. By placing an elastomeric spacer (e.g., PDMS orKapton film) between the conductive glass substrates, the maximum heightof the photofabricated structure can be defined and precise fields canbe applied to polarize the BTO nanoparticles. After activating thecomposite materials the fabricated films can either be left on the glassslides for testing and characterization, removed to create free-standingstructures, or transferred to other substrates for further integration.If the photo-printing is carried out on a substrate that has weakinteractions with the PEGDA composite (e.g., hydrophobic surface), thestructured films can roll up to make higher order structures, as shownin FIG. 3B. For example, the exemplary microtube shown in FIG. 3B is oneexample where a honeycomb pattern is projected onto the nanoparticlecomposite solution and after polymerization is removed from thesubstrate which causes the sheet to roll up into well-defined tubules.This exemplary process should be controllable by fabricating bilayerswith different thermal expansion coefficients, densities, or latticeparameters which would govern the diameter of the tube and extent of therolling.

The exemplary implementation included characterizing the piezoelectricproperties of the materials by applying specific loads to thepolymerized films and measuring the electrical outputs. As expected,there was a significant enhancement in the cross-linked films thatcontain the TMSPM linker (no CNTs) compared to the piezoelectricmaterial without the linker but with CNTs (1%), for example. In fact,under similar loads (e.g., 1.44 N normal to plane of the film), thematerial with the grafted nanoparticles displayed a greater than threetimes boost in the piezoelectric output over the CNT composites, asshown in the data plot of FIG. 4A. There was no response in filmsfabricated with 1) untreated BTO nanoparticles and no CNTs or 2)unpolarized materials containing TMSPM. Quantifying the piezoelectricresponse of the 10% BTO loaded CNT composites and TMSPM-graftedmaterials gave effective piezoelectric coefficient (d₃₃) values of 13±2pC/N and 39±3 pC/N, respectively. These exemplary values for the graftedmaterials are already exceeding that of pure polymers such as PVDF,which may warrant further investigation into the upper limit of thephotoliable materials and systematically studying the dependence of thepiezoelectric properties on nanoparticle composition, polymer matrix,nanoparticle size, and linker chemistry. The large increase in thepiezoelectric coefficient is directly related to the mechanicalinterface between the BTO surface and PEGDA matrix which aids in themechanical-to-electrical energy conversion process by efficientlytunneling the strain of the polymer chains to the piezoelectriccrystals. Although the piezoelectric properties of the piezoelectricmaterials were lower than BTO monolithic ceramics (e.g., greater than200 pC/N), the piezoelectric materials are performing with a much lowerdensity of active material.

The exemplary implementations included analyzing the piezoelectriccoefficient as a function of BTO mass loading, as shown in FIG. 4B. Thedata plot of FIG. 4B shows a clear trend towards higher d₃₃ values asthe nanoparticle density increases. This upward trend should continue toincrease and likely peak at some higher mass loading, but currently itis difficult to photo-fabricate films above 10% loading due to the largeextinction coefficients. To address this, smaller nanoparticles withweaker light-matter interactions can be used.

FIG. 4A shows a data plot of the voltage response of photopolymerizedpiezoelectric materials cycled under a 1.44 N load applied perpendicularto the surface of the film. The films with TMSPM and no CNTs (402)showed an approximately three fold increase in the piezoelectric outputcompared to films with CNTs and no TMSPM (401). FIG. 4B shows a dataplot of the effective piezoelectric modulus (d₃₃) as a function of BTOmass loading.

Methods Implemented in the Exemplary Implementations

Barium titanate (BTO) nanoparticle synthesis: BTO nanoparticles weresynthesized by hydrothermal methods. The exemplary precursors for thereaction included barium hydroxide monohydrate (Ba(OH)₂—(H₂O) [SigmaAldrich, 98%), titanium butoxide (Ti[O(CH)₂CH₃]₄ [Ti-butoxide; SigmaAldrich, 97%], and diethanolamine (NH(CH₂CH₂OH)₂ [DEA, FisherScientific, laboratory grade). First, 25 mmol of Ti-butoxide was addedto 10 mL of ethanol followed by the addition of 3.5 mL of ammonia. TheTi-butoxide solution was then mixed with the Ba-hydroxide solution whichcontained 37.5 mmol of Ba-hydroxide in 12.5 mL DI water. The DEA (2.5mL) was then added to the solution to help control the size of thenanoparticles. The final solution was transferred to Teflon linedstainless steel reactor and the reactor was kept in oven at 200° C. for16 hours. After the reaction, the reactor was cooled down to roomtemperature and the particles were cleaned 10 times with a vacuum filterusing ethanol and DI water. The final product was dried at 80° C. for 24hours. FIG. 9 shows the x-ray diffraction pattern of the as-made BTOnanoparticles (901) and a reference BaTiO₃ pattern (902). The x-raydiffraction spectra indicate that the nanoparticles have a strongtetragonal phase given their c/a ratio of 1.007. The spectra wererecorded using a Bruker D9 X-ray diffractometer.

Preparation of PEGDA and BTO nanoparticle materials: Prior to mixing theBTO nanoparticles with the polyethylene glycol diacrylate (PEGDA)solutions, the dried nanoparticles were functionalized with3-trimethoxysilylpropyl methacrylate (TMSPM) using similar graftingstrategies to those carried out on silica surfaces. The TMSPM solutionincluded 1 mL TMSPM dissolved in 50 mL of ethanol and mixed with anacetic acid solution (e.g., 1 mL acetic acid in 9 mL of DI water). TheBTO nanoparticles (approximately 6 g) were then added to the TMSPMsolution and sonicated for 24 hours. After the surface functionalizationstep, the particles were cleaned with copious amounts of ethanol andwater and dried. FIG. 10 shows chemical structure of TMSPM (1001), andthe FTIR spectra of pure TMSPM (1002), as-made BTO nanoparticles (1003),and TMSPM-grafted BTO nanoparticles (1004). FTIR measurements were takenon as-made and freshly functionalized nanoparticles. The bands at2862-2882 cm⁻¹ are attributed to C—CH₃ and O—CH₃ groups and the band at1720 cm¹ is attributed to the C═O group. The samples were washed withcopious amounts of ethanol and water prior to taking spectra and providegood evidence (along with the enhanced piezoelectric properties) thatthe TMSPM is grafted to the surface of the BTO nanoparticles. Thespectra were recorded using a Spectrum Two spectrometer (Perkin Elmer).To prepare the BTO-loaded PEGDA solutions, appropriate BTO:PEGDA weightratios were used to achieve the desired mass loading and the sampleswere sonicated for greater than 24 hours prior to photopolymerization.

Optical printing and film preparation: The optical printing cellsincluded cover glass slides coated with 100 nm of ITO deposited bymagnetic sputtering. The electrodes were covered with approximately 1 μmof polymethyl methacrylate (PMMA) to prevent shorting. A photoinitiatorsuch as 2,2-dimethoxy-2-phenylacetophenone (DMPA) or2,2-dimethoxy-1,2-di(phenyl)ethanone (Irgacure 651) was added to thePEGDA composites at a concentration of 1 wt %. The PEGDA composite wasthen placed between the two electrodes using a 25 μm Kepton film spacerand the polymer could be polymerized using 365 nm light from an LED (forDPP) or a hand held UV lamp (for film preparation). The power of thehand held lamp was much lower than the LED which required longerexposure times (minutes) to photopolymerize. Electrical wires wereconnected to the electrodes using silver epoxy and the photopolymerizedsamples were electrically poled at a field of approximately 12 MV/m at120° C. for 24 hours. The piezoelectric properties of the polymers werecharacterized using a commercially available force sensor (Tekscan,A201) and a home-built charge amplifier. FIGS. 12A-12B show a schematicof the FlexiForce sensor (Tekscan) circuit and the calibration curve. Asshown in FIG. 12A, the piezoelectric polymer was placed in between twoPDMS pieces prior to placing on the force sensor (1201) to protect thematerials during the mechanical test and to distribute the load equallyover the active area of the piezoelectric. However, it was found thatthere are minimal differences between similar loads with differentcontact areas. The output voltage of the sensor could be tuned bychanging the supply voltage and the feedback resistor, R₁(1202). Thecapacitor, C₁ (1203), was used as the bypass capacitor. As shown in FIG.12B, under the most sensitive set-up (−5 V supplied and R₁=10 kohm) thesensor shows a linear relationship with respect to load above 2 N and anexponential relation below 2 N. FIG. 8 shows a schematic of the chargeamplifier used in the piezoelectric experiments. Charge generated fromthe piezoelectric polymer is transferred to the reference capacitor, C₁(801), and produces an output voltage, V_(OUT) (802), that is equal tothe voltage across C₁ (801). The V_(OUT) (802) can be expressed asV_(c)=−QO_(GENERATED)/C₁. In the experiments a 100 pF capacitor (and 20Mohm feedback resistor, R₁ (803)) was used as the reference capacitorallowing the piezoelectric coefficient, d₃₃, to be calculated fromd₃₃=V_(OUT)×100 pF/F_(APPLIED).

The disclosed technology includes a 3D printing process for fabricatingpiezoelectric materials. In some implementations, for example, byembedding piezoelectric nanoparticles in a photoliable polymer solution,optical images can be projected onto the polymer solution causingphotopolymerization to occur wherever the medium is exposed. To enhancethe mechanical-to-electrical conversion process of the piezoelectricmaterials, the surface of the piezoelectric nanoparticles can bechemically modified with linker molecules that could cross-link with thepolymer matrix under light exposure. This can allow the nanoparticles tobecome encapsulated by the polymerization process and grafted to thesolidified polymer network. The exemplary resolution is affected by thespot size of the light source which can be significantly reduced usingdifferent light sources, wavelengths, nonlinear optical effects such asTPA, and/or higher focusing components. With the potential to printvirtually any 3D piezoelectric shape, while maintaining a strongpiezoelectric coefficient and biocompatible properties, the disclosedtechnology can find immediate application in integratedmicro/nanoelectromechanical systems, sensors, bio-imaging, and energyscavenging.

In some aspects, a method of the disclosed technology includesphotopolymerizing a photoliable polymer solution (e.g., acrylatedpolyethylene glycol—PEGDA) containing piezoelectric nanoparticles (e.g.,such as BTO, PZT, ZnO, or NaNbO₃), like that shown in FIG. 5. In oneaspect, a method includes photopolymerizing a selected portion of a twodimensional plane in a sample (501) of a photoliable polymer solutioncontaining piezoelectric nanoparticles to form a layer of apiezoelectric material (502), the photopolymerizing including directinglight from a light source based on a pattern design in the selectedportion of the photoliable polymer solution; and moving one or both ofthe sample and the directed light to photopolymerize another selectedportion of another two dimensional plane in the sample to form anotherlayer of the piezoelectric material. Similar strategies can also be usedto incorporate pure piezoelectric polymers, e.g., such as PVDF, bymaking blends of the photoliable polymer solution with the piezoelectricpolymer. Under light exposure the carbon-carbon double bonds of thepolymer chains cross-link forming a covalently linked network. Toachieve stronger binding to the polymer matrix, and enhancemechanical-to-electrical energy conversion, the surface of thepiezoelectric nanoparticles can be chemically modified with linkerscontaining acrylate end groups, like that shown in FIG. 6B. An exampleof a silane-based compound is 3-trimethoxysilylpropyl acrylate (TMSPA).After mixing the modified nanoparticles with the polymer solution, thematerials can be printed using various photopolymerization process suchas microstereolithography (μSL) or two-photon polymerization (TPP) orDigital Projection Printing (DPP). As an example of this, FIG. 6C showsa simple honeycomb and star shape created using a μSL setup thatprojects 365 nm light onto a PEGDA/BTO solution deposited on an indiumtin oxide (ITO) surface. The exemplary piezoelectric properties of thefilm is activated by applying an electric field greater than thecoercive field of the BTO nanoparticles (greater than 10 V/μm). FIG. 6Dshows exemplary results that depict the piezoelectric voltage of thegrafted nanoparticles enhanced by approximately 4 times, e.g., ascompared to materials that contain carbon nanotubes (CNTs). The additionof CNTs (without any other linkage between the nanoparticles and polymermatrix) has been shown to significantly increase outputs overnanoparticle-only materials.

FIG. 6A-6D shows exemplary electron micrograph images and illustrativediagrams showing exemplary piezoelectric materials produced by thedisclosed methods. FIG. 6A shows an exemplary electron micrograph of BTOnanoparticles grown via a hydrothermal process. FIG. 6B shows anillustrative diagram depicting the exemplary piezoelectric material withBTO nanoparticles (602) grafted to a PEGDA matrix (601). FIG. 6Bincludes an inset showing a zoomed area depicting the TMSPA linker (603)covalently linked to the nanoparticle surface (602) and PEGDA matrix(601). FIG. 6C shows exemplary scanning electron micrographs (SEM) of ahoneycomb and star shape (with a scratch to show thickness) fabricatedout of a PEGDA/BTO materials using μSL. FIG. 6D shows exemplary resultsof the voltage response of photopolymerized materials under anapproximately 5 N load with CNTs and no TMSPA (604), and with TMSPA andno CNTs (604), and showing that there is no response without CNTs andTMSPA.

Using the disclosed techniques, other more complex 3D piezoelectricstructures can be fabricated. For example, FIG. 7 shows an exemplaryfingerprint structure created using the disclosed technology. Complex 3Dpiezoelectric structures can be created to facilitate one or more ofbiocompatible, mechanically flexible, size scalable, shape tunable, andhighly responsive piezoelectric materials.

The disclosed technology can also be implemented to print variable 3Dpiezoelectric shapes, while maintaining a strong piezoelectriccoefficient and biocompatible properties, and thus is applicable tobiomimetic materials (e.g., artificial skin, tympanic membrane),integrated micro/nanoelectromechanical systems (MEMS/NEMS, e.g., such asmechanical actuators), sensors (e.g., acoustic detection), bio-imaging(e.g., high resolution, compact ultrasonic imaging instruments), invitro energy scavenging, and biometrics and security, among others.

FIGS. 13A-13F illustrate some of the suitable applications using the 3Dpiezoelectric materials and the disclosed technology in applications.For example, as shown in FIG. 13A, 3D piezoelectric material can be usedto determine biometrics of an individual. In such an application, 3Dpiezoelectric material can be used, for example, in a sensor attached toan individual. When the individual moves, the 3D piezoelectric materialgenerates a direct piezoelectric effect that can be measured andanalyzed. In another application, the 3D piezoelectric material can beused for generating biomimetic materials such as artificial skin ortympanic membrane. In the case of the tympanic membrane, vibrations inthe ear canal can be converted to electrical signals by the 3Dpiezoelectric material.

In FIG. 13B, 3D piezoelectric material can be used inmicro/nanoelectromechanical systems (MEMS/NEMS, e.g., such as mechanicalactuators). Based on an electrical signal, the 3D piezoelectric materialcan generate a mechanical response.

In FIG. 13C, 3D piezoelectric material can be used to harvest energy.For example, if a 3D piezoelectric material is directly or indirectlyconnected to an individual, any movement generated by the individual cangenerate an electrical response in the 3D piezoelectric material. Suchan electrical response can be used, for example, to charge a battery ina wearable device, such as a FitBit or Apple Watch, and in medicaldevices.

In FIG. 13D, 3D piezoelectric material can be used inmicro/nanoelectromechanical systems to convert electrical stimuli tomechanical strain.

In FIG. 13E, 3D piezoelectric material can be used for acousticmanipulation. For example, 3D piezoelectric material can be used inspeakers to convert electrical stimuli to mechanical strain.

In FIG. 13F, 3D piezoelectric material can be used in acoustic imagingsystems, for example, in a transducer to convert electrical stimuli tomechanical strain, and vice versa.

While this patent document contains many specifics, these should not beconstrued as limitations on the scope of any invention or of what may beclaimed, but rather as descriptions of features that may be specific toparticular embodiments of particular inventions. Certain features thatare described in this patent document in the context of separateembodiments can also be implemented in combination in a singleembodiment. Conversely, various features that are described in thecontext of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. Moreover, the separation of various system components in theembodiments described in this patent document should not be understoodas requiring such separation in all embodiments.

Additional enhancements and variations can be made based on what isdescribed and illustrated in this patent document.

What is claimed are techniques and structures as described and shown,including:
 1. A method to fabricate a piezoelectric material,comprising: photopolymerizing a selected portion of a two dimensionalplane in a sample of a photoliable polymer solution containingpiezoelectric nanoparticles to form a layer of a piezoelectric material,the photopolymerizing including directing light from a light sourcebased on a pattern design in the selected portion of the photoliablepolymer solution; and moving one or both of the sample and the directedlight to photopolymerize another selected portion of another twodimensional plane in the sample to form another layer of thepiezoelectric material.
 2. The method as in claim 1, wherein the formedpiezoelectric material is formed into a predetermined shape based on apattern design.
 3. The method as in claim 1, wherein the photoliablepolymer solution includes at least one of a poly(ethylene glycoldiacrylate), poly(methyl methacrylate), poly(acrylic acid), orpoly(lactic acid).
 4. The method as in claim 1, wherein thepiezoelectric nanoparticles include one or more of barium titanate(BTO), lead magnesium niobate-lead titanate (PMN-PT), lead zirconatetitanate (PZT), zinc oxide (ZnO), or sodium niobate (NaNbO₃).
 5. Themethod as in claim 1, wherein the light source includes at least one oflaser or light emitting diode (LED).
 6. The method as in claim 1,wherein the light source emits ultraviolet light.
 7. The method as inclaim 1, wherein the formed piezoelectric material exhibits propertiesincluding biocompatible, mechanically flexible, size scalable, shapetunable, directly piezoelectrically responsive, or converselypiezoelectrically responsive.
 8. The method as in claim 1, furthercomprising: adding a linker molecule to the photoliable polymer solutionto covalently link the piezoelectric nanoparticles to a polymer matrixin the photoliable polymer solution.
 9. The method as in claim 8,wherein the linker molecule includes 3-trimethoxysilylpropylmethacrylate (TMSPM).
 10. The method as in claim 8, wherein the linkermolecule includes 3-trimethoxysilylpropyl acrylate (TMSPA).
 11. Themethod as in claim 1, further comprising: adding amechanical-to-electrical enhancer to the photoliable polymer matrixwherein the mechanical-to-electrical enhancer includes carbon nanotubes(CNTs).
 12. The method as in claim 1, further comprising: activating thepiezoelectric material by applying an electric field larger than acoercive field of the piezoelectric nanoparticles.
 13. A devicecomprising a piezoelectric material structure which includes: athree-dimensional structure; and a plurality of piezoelectricnanoparticles grafted to the three-dimensional structure; and whereinthe piezoelectric material structure is operable to generate an electriccharge in response to an applied mechanical force and to generate amechanical force in response to an applied electrical signal; andwherein the piezoelectric material structure is fabricated using alight-assisted polymerization technique by causing crosslinking in aphotoliable polymer solution.
 14. The device as in claim 13, furthercomprising: a linker molecule that covalently links the piezoelectricnanoparticles to a polymer matrix in the piezoelectric materialstructure.
 15. The device as in claim 13, wherein the piezoelectricnanoparticles include one or more of barium titanate (BTO), leadmagnesium niobate-lead titanate (PMN-PT), lead zirconate titanate (PZT),zinc oxide (ZnO), or sodium niobate (NaNbO₃).
 16. The device as in claim13, wherein the piezoelectric material structure exhibits biocompatible,mechanically flexible, size scalable, shape tunable, directlypiezoelectrically responsive, or conversely piezoelectricallyresponsive.
 17. The device as in claim 13, further comprising: a linkermolecule that covalently links the piezoelectric nanoparticles to apolymer matrix in the piezoelectric material structure.
 18. The deviceas in claim 17, wherein the linker molecule includes3-trimethoxysilylpropyl methacrylate (TMSPM).
 19. The device as in claim17, wherein the linker molecule includes 3-trimethoxysilylpropylacrylate (TMSPA).
 20. The device as in claim 13, further comprising amechanical-to-electrical enhancer, wherein the mechanical-to-electricalenhancer includes carbon nanotubes (CNTs).
 21. The device as in claim13, comprising a biometric sensor based on the piezoelectric materialstructure operable to convert mechanical stress to electrical signals.22. The device as in claim 13, comprising a sensing circuit coupled tothe piezoelectric material structure to receive an electrical signalproduced by the piezoelectric material structure in response to amechanical stress applied thereto.
 23. The device as in claim 13,comprising a microelectromechanical or nanoelectromechanical devicebased on the piezoelectric material structure operable to convert one ormore of mechanical stress to electrical signals or electrical signals tomechanical stress.
 24. The device as in claim 13, wherein the device isa wearable device operable to convert mechanical stress to electricalsignals to charge a battery.
 25. The device as in claim 13, comprisingan acoustic speaker based on the piezoelectric material structureoperable to convert electrical signals to mechanical stress for audiosound production.
 26. The device as in claim 13, comprising an acousticimaging system based on the piezoelectric material structure operable toconvert mechanical stress to electrical signals in receiving ultrasoundimage signals for imaging reconstruction or to convert electricalsignals to mechanical stress in transmitting ultrasound signal forimaging.
 27. A method to fabricate a device based on a piezoelectricmaterial, comprising: directing an optical beam to a photoliable polymersolution containing piezoelectric nanoparticles; modulating andcontrolling the optical beam in a way that causes photopolymerizing ofselected portions of the photoliable polymer solution to form a threedimensional piezoelectric material structure; forming one or moreelectrical contacts to the three dimensional piezoelectric materialstructure to construct the device to apply an electrical control to thethree dimensional piezoelectric material structure, or to receive anelectrical response from the three dimensional piezoelectric materialstructure under a stress.
 28. The method as in claim 27, wherein theoptical beam is modulated by a two-dimensional spatial light modulatorto carry a spatial pattern that forms part of the shape of the threedimensional piezoelectric material structure.
 29. The method as in claim28, wherein the two-dimensional spatial light modulator includes adigital micromirror-array device.
 30. The method as in claim 28, whereinthe part of the shape of the three dimensional piezoelectric materialstructure is a layer of the three dimensional piezoelectric materialstructure, and wherein the modulating and controlling the optical beaminclude modulating the optical beam at to carry a spatial pattern of onelayer of various layers of the three dimensional piezoelectric materialstructure and to carry different spatial patterns of different layers ofthe three dimensional piezoelectric material structure at differenttimes to cause crosslinking at different depths of the photoliablepolymer solution.
 31. The method as in claim 27, wherein the opticalbeam is at a UV wavelength that can directly cause crosslinking in thephotoliable polymer solution.
 32. The method as in claim 27, wherein theoptical beam is at a longer wavelength than a UV wavelength that candirectly cause crosslinking in the photoliable polymer solution, and thecontrolling of the optical beam includes controlling an optical power ofthe optical beam at a selected location in the photoliable polymersolution to cause a nonlinear optical two-photon absorption thattriggers crosslinking in the photoliable polymer solution.
 33. Themethod as in claim 27, wherein the photoliable polymer solution includesat least one of a poly(ethylene glycol diacrylate), poly(methylmethacrylate), poly(acrylic acid), or poly(lactic acid).
 34. The methodas in claim 27, wherein the piezoelectric nanoparticles include one ormore of barium titanate (BTO), lead magnesium niobate-lead titanate(PMN-PT), lead zirconate titanate (PZT), zinc oxide (ZnO), or sodiumniobate (NaNbO₃).